The use of polymer beads with diameters in the range of 0.5 to 20 .mu.m as spacer particles is well known. The use of such polymeric beads as spacer in photosenstive materials is disclosed in e.g. U.S. Pat. No. 4,614,708, U.S. Pat. No. 5,252,445, U.S. Pat. No. 5,057,407, EP 281 928 etc. Also in the manufacture of polymeric sheets or webs intended for, e.g., use as packaging material the use of polymeric spacer particles is known, e.g. in DE-OS 39 30 141 polymeric spacers useful in the manufacture of polymeric webs or sheets for packaging are disclosed.
In both uses said polymeric spacer particles improve the antistatic properties of the material and prevent blocking of the sheet or web material.
Given the different applications of polymeric spacing particles, it is clear that such particles have to fulfil several requirements. Polymeric spacer particles have, e.g., to be insoluble in organic solvents, insoluble in water but dispersable in water or at least in hydrophilic dispersing media. When used in the production of polymeric sheet or web materials the polymeric spacer particles have to be heat resistant, even in an atmosphere containing oxygen. Apart from the physical and chemical properties, polymeric spacer particles are needed in a wide range of average particle sizes, ranging from 0.1 .mu.m to 20 .mu.m.
In EP 466 982 a method is provided for preparing finely divided solid polymer beads that are insoluble in water, in organic solvents, and in mixtures of water and organic solvents, that have an average particle size in the range of from 0.5 to 5 .mu.m and have a glass transition temperature of at least 40.degree. C. These particles have also proven to be heat stable, up to about 220.degree. to 250.degree. C. However this method is rather complicated since it comprises four consecutive steps and with this method it is very difficult to produce polymer beads with average particles size.gtoreq.5 .mu.m.
In DE 39 30 141, a two step reaction for the manufacture of polymer beads with average particle size between 3.5 .mu.m and 10 .mu.m is described. The spacer particles do show a narrow size distribution and are insoluble in organic solvents as well as heat resistant. However with the teachings of this disclosure, it is only possible to produce the spacing particles in a non-aqueous organic solvent mixture. In DE 39 30 141 it is disclosed to use organic solvent mixture comprising at most 5% water. Production procedures using organic solvents are from the view point of solvent containment and ecology less desirable. On the other hand, with the procedure disclosed in DE 39 30 141 it is very difficult to produce polymer beads with average particle size.gtoreq.3.5 .mu.m.
In EP-B 080 225 and the corresponding U.S. Pat. No. 4,861,818 a single step method for the formation of polymer beads in an aqueous environment is disclosed, but the polymer beads produced according to the method of that disclosure are not cross-linked and are insufficently insoluble in organic solvents and insufficiently heat resistant.
There is still a need for a simple method (single step reaction) to produce polymer beads that are insoluble in organic solvents and heat resistant.